Adds the ability to enable / disable sorting of BIO requests queued within

CAM. This can significantly improve performance particularly for SSDs
which don't suffer from seek latencies.

The sysctl / tunable kern.cam.sort_io_queues provides the systems default
setting where:-
0 = queued BIOs are NOT sorted
1 = queued BIOs are sorted (default)

Each device gets its own sysctl kern.cam.<type>.<id>.sort_io_queue
Valid values are:-
-1 = use system default (default)
0 = queued BIOs are NOT sorted
1 = queued BIOs are sorted

Note: Additional patch will look to add automatic use of none sorted queues
for none rotating media e.g. SSD's

Reviewed by:	scottl
Approved by:	pjd (mentor)
MFC after:	2 weeks

1 files changed, 3 insertions, 0 deletions

diff --git a/sys/cam/cam.h b/sys/cam/cam.h
index a3e191f0fce3..20fc7a37dea6 100644
--- a/sys/cam/cam.h
+++ b/sys/cam/cam.h
@@ -228,6 +228,9 @@ struct cam_status_entry
 
 extern const struct cam_status_entry cam_status_table[];
 extern const int num_cam_status_entries;
+#ifdef _KERNEL
+extern int cam_sort_io_queues;
+#endif
 union ccb;
 
 #ifdef SYSCTL_DECL	/* from sysctl.h */